In an automotive electrical system, it is often critically important that devices directly connected to the vehicle battery limit the current they consume during low power scenarios, such as when vehicle ignition is off. This is to prevent unnecessary discharge of the vehicle battery while the engine is not running. In order to achieve this goal in an automotive electronic device, it is common practice to place one or more power switches in series between the battery input of the power system and the power inputs of downstream circuitry in order to allow them to be fully powered off during low power scenarios.
The power supply line for integrated circuits and downstream loads is typically decoupled with a several distributed capacitances to ground. That distributed capacitance, along with any other lumped parallel capacitance, is electrically equivalent to a short circuit during the occurrence of high frequency voltage transients on the power supply line, such as the fast turn-on transition caused by an upstream power switch. Uncontrolled inrush current into distributed capacitance can cause damage to the power switch, upstream components or the capacitors themselves, and can also potentially blow an upstream fuse.
When using a voltage-controlled semiconductor-based switch, whose output is controlled based on the difference between its control node and its input (such as a P-channel MOSFET), it is a common practice to introduce capacitive feedback from the output of the switch to the control node of the switch so that the slew rate of the voltage ramp that it applies to the downstream distributed capacitance during its turn-on transition is precisely controlled, thereby limiting the inrush current that is conducted into the downstream distributed capacitance while providing a predictable turn-on time. For the “off” state, a resistance is typically connected between the gate and the source of the MOSFET. An unfortunate consequence of using such a slew-rate controlling capacitance connected from output to control node of a semiconductor switch, along with a resistor to effectuate the “off” state, is that the off-state resistor and slew-rate controlling capacitance can create a mechanism that enables or turns on the semiconductor switch if an upstream voltage transient occurs by introducing a transient difference between the input and the control node. Thus the switch with output-connected capacitive slew-rate control feedback may not always fulfill its duty of blocking downstream current when it is expected to be disabled.
An apparatus and method for effectively suppressing the propagation or transmission of voltage transients through a semiconductor switch having a slew rate that is controlled using output-connected capacitive feedback, when the switch is expected to be disabled, would be an improvement over the prior art.